The present invention relates to drilling techniques for oil, gas, water, geothermal or analogous wells. More precisely, the invention relates to an additive, and to compositions including the additive, for cement slurries, more particularly for cementing a casing in an oil well or the like.
After drilling an oil well or the like, a casing or a coiled tubing is lowered into the well and cemented over all or a portion of its length. Cementing can in particular prevent fluids being exchanged between the different formation layers through which the well passes, it can prevent gas from rising via the annular space surrounding the casing, or it can limit the ingress of water into the production well. Of course, it also has the principal aim of consolidating the well and protecting the casing.
While a cement slurry is being prepared, then injected into the well, and finally positioned in the zone to be cemented, it must have relatively low viscosity and practically constant rheological properties. In contrast, once it is in position, an ideal cement would rapidly develop high compressive strength so as to enable other work in the well under construction to be resumed, in particular to enable drilling to be continued.
In practice, practically all cementing slurries are formulated with an additive which retards setting of the cement, normally known as a retarder. The most widely used retarders are lignosulfates, hydroxycarboxylic acids such as citric acid, glucoheptonic acid or gluconic acid, saccharides or polysaccharides such as carboxymethylhydroxyethyl cellulose, and organophosphates.
In practice, selecting a retarder depends on the temperature at the well bottom, the cement slurry circulation temperature, and the presence or absence of other additives with which the retarders may be incompatible. The majority of known retarders are effective only in a relatively narrow temperature range, a fact which is more critical as the temperatures to which the cement slurries are subjected are not always precisely known. A further difficulty is high sensitivity to variations in concentrations of retarder or of other additives, and occasionally also to the cements used.
Under such conditions, formulating a cement slurry which is suitable for every eventuality remains a particularly difficult art, all the more so since oil well cements are, by their very definition, used on sites which are usually far from the facilities of an industrial laboratory and which generally do not have access to the complete range of available additives.
The present invention aims to provide a novel retarding system which is suitable for low/medium temperature applications, namely typically 70xc2x0 C. to 140xc2x0 C., and which is compatible with additives which are currently used for oilfield cements such as latexes, chemically cross-linked polyvinyl alcohol type fluid loss control agents (in particular those described in U.S. Pat. No. 5,594,050) and for which the effect on a cement slurry is readily predictable, and in particular which has low sensitivity to variations in cement quality.
This aim is satisfied in the present invention by a system constituted by a solution of a phosphonate selected from derivatives of methylene phosphonic acid, and a phosphate.
The phosphates may be mono-phosphates (ortho-phosphates PO4, meta-phosphates PO3) or acyclic poly-phosphates (pyrophosphates P2O74, tripolyphosphates P3O105), or cyclic poly-phosphates. Salts can also be used, for example, preferably sodium or potassium salts, or the acids (if they exist) of the following compounds: orthophosphoric acid H3PO4, sodium dihydrogen phosphate NaH2PO4, sodium monohydrogen phosphate Na2HPO4, trisodium phosphate Na3PO4, pyrophosphoric acid H4P2O7, sodium tripolyphosphate Na5P3O10, and sodium cyclotrimetaphosphate Na3P3O9.
The preferred retarding system of the invention is obtained with a calcium and sodium salt of ethylenediamine-N,N,Nxe2x80x2,Nxe2x80x2-tetrakis(methylene) phosphonic acid or the pentasodium salt of ethylenediamine tetra(methylenephosphonic) acid, associated with an orthophosphate.
The phosphonate to phosphate weight ratio is preferably in the range 2 to 4, more preferably in the range 3 to 3.5.
The retarding system of the invention is suitable for applications between 50xc2x0 C. and about 140xc2x0 C. In a variation of the invention, the retarding system also comprises a retarder booster which can extend the range of application of the retarder of the invention to medium temperatures. For reasons of increased compatibility with other additives, and in particular fluid loss control additives, it is preferable to use as a retarder booster a mixture of lignosulfates and hydroxycarboxylic acids (such as gluconates), but other conventional hydroxycarboxylic acid-based retarder boosters or lignosulphates can also be used. It should be noted that these retarder boosters are themselves retarders but of quite low efficiency and thus are usually used in combination with other retarders. In this optimised variation of the invention, the retarding system is constituted by 40% to 45% of orthophosphoric acid, 10% to 15% of the calcium and sodium salt of ethylenediamine-N,N,Nxe2x80x2,Nxe2x80x2-tetrakis(methylene)phosphonic acid and 40% to 50% of retarder booster, the percentages being by weight.
The retarding system of the invention can also contain a biopolymer which can improve the rheology of the cement slurry by minimizing settling problems for systems with a high concentration of the retarding system.
In contrast to the numerous conventional retarders, the retarding system of the invention is advantageously compatible with different types of fluid loss control agents or gas migration agents, in particular with latexes, and can be used in slurries in which seawater is used as the mixing water.